A heat recovery steam generator (HRSG) is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process (cogeneration) or used to drive a steam turbine (combined cycle). Gas turbines with heat-recovery—steam generators (HRSGs) can be found in nearly all chemical process industries (CPI) plant. They can be operated in either the cogeneration mode or in the combined-cycle mode. In the cogeneration mode, steam produced from the HRSG is mainly used of process applications, whereas in the combined-cycle mode, power is generated via a steam turbine generator.
FIGS. 1(A) and 1(B) is a cross-sectional schematic diagram that depicts one embodiment of a HRSG 100. FIG. 1(A) reflects one side view of the HRSG while the FIG. 1(B) depicts another side view of the HRSG. Both FIGS. 1(A) and 1(B) are not drawn to scale and do not have a 1:1 correspondence to each other even despite being different side views of the same design.
The heat recovery steam generator 100 is located downstream of a turbine (not shown) and receives an exhaust stream from the turbine. The exhaust stream flows through the connecting duct 132 and HRSG inlet duct 130. The exhaust stream passes through a grate-like array of tubes 150, 160 (hereinafter tube bundles) located in heat exchanger space 120 of the HRSG 100, where it transfers its heat to fluids contained in the tube bundles.
The following description applies to all tube bundles but particularly details the tube bundle 160. The tube bundles 160 comprise a first header 110 and an optional second header 112 that are contained in vestibules 122 and 124 respectively. The vestibules 122 and 124 are enclosures that lie above and below the exhaust stream flow path (the heat exchanger space 120) and therefore do not encounter the hot gases of the exhaust stream. The vestibules 122 and 124 are therefore at a lower temperature than the heat exchanger space 120. The portions of the tube bundle located in the vestibules 122 and 124 are therefore subjected to lower temperatures than the portions of the tube bundle located in the heat exchanger space 120.
The tube bundle 160 comprises a plurality of tubes 102, 104, 106 and 108 and so on that are more or less vertically disposed in the HRSG and that are in fluid communication with the headers 110 and 112 respectively. While the FIG. 1 depicts the tubes 102, 104, 106 and 108 as being located in the plane of the paper, each tube bundle comprises a plurality of tubes that extend in a direction perpendicular to the plane of the paper forming a series of tube arrays through which the exhaust stream can pass and transfer its heat. This is depicted in the FIG. 1(B). As can be seen in the FIGS. 1(A) and 1(B), a plurality of tubes 102, 104, 106 and 108 lie within the space 120 where the exhaust stream contacts them. A fluid is contained in the tube bundle 160.
In the HRSG 100 shown in the FIG. 1(A), the entire tube bundle comprising tubes 102, 104, 106 and 108 along with the second header 112 are suspended from the header 110. The entire weight of the tube bundle 160 and any supporting equipment is borne by the header 110, portions of the tube 116 that may connect the header 110 to an external header (not shown), upper portions of the tubes 102, 104 106 and 108, and by welds (202, 204) in the tube 116 or the welds (208, 210) in the tubes 102, 104, 106 and 108. As detailed below this load plays an important role in the deterioration of the welds during the life cycle of the HRSG.
The welds that bond weld sections 202, 204 in the tube 116 are generally conducted to bring together different tube sections in order to produce tubes of a variety of different shapes (not shown here). Tube sections present in the vestibules 122 and 124 are generally comprised of an expensive austenitic steel and an alloy steel. These sections of different materials are bonded together to produce the tube 116.
Fusion of austenitic to ferritic grade metal is inherently an incompatible weld and by definition a dissimilar metal weld (DMW). In general, the best practices of those skilled in the art of DMW management is to avoid thick walled DMW's by placing the DMW on a relatively small diameter tube, which in turn results in a far thinner wall than that required at location 202 and 204. Additionally, it is desirable to minimize all stresses (thermal, mechanical, static, and the like) at the DMW to ensure prolonged life. Lastly, accessibility for serviceability and repair is also useful to help monitor DMW's and maximize equipment uptime.
The prior art of FIG. 1A has some advantages in that accessibility to the DMW is good hence monitoring can be ensured. Moreover, many of the stresses acting on the DMW can be lessened by means of proper design of tube expansion loops (tube 116 can contain substantial expansion loops in proportion to the calculated stresses borne from thermal growth, and the like). What the prior art solution presented in FIG. 1A with the DMW at 202 and 204 lacks is a thin walled dissimilar weld—the current configuration often yields wall thicknesses at the DMW in excess of 25.4 mm (1 inch)—particularly for high pressure circuits. Hence, while management of mechanical stresses acting on the DMW is controllable and accessibility to the DMW's 202 and 204 depicted in FIG. 1A remains good the fundamental design shortcoming remains due to the thick walled DMW that cannot be overcome.
On the other hand, with regard to placement of the dissimilar metal welds (DMW) at location 208 and 210 (present in the space 120 where the exhaust stream contacts the tubes 102, 104, 106 and 108), the higher temperatures in the space 120 that the dissimilar weld is subjected to along with the load of the structure acting on the weld causes rapid weld deterioration. Moreover, access is significantly curtailed with this solution, hence any repair requires substantial investment. This means for all intents and purposes that if any failure is detected on any one DMW of 208 and 210 within a tube bundle a wholesale replacement is generally undertaken from an end user's perspective. It is therefore desirable to relieve the load on the DMW imparted by the tube bundles and also position the DMW in a highly accessible location; doing so reduces maintenance and down time.